Silicon carbide single-crystal substrate and method for manufacturing same

ABSTRACT

A silicon carbide single-crystal substrate includes a first surface, a second surface opposite to the first surface, and a peripheral edge portion sandwiched between the first surface and the second surface. A plurality of grinding traces are formed in a surface of the peripheral edge portion. A chamfer width as a distance from an outermost peripheral end portion of the peripheral edge portion to one of the plurality of grinding traces which is located on an innermost peripheral side of the peripheral edge portion in a direction parallel to the first surface is not less than 50 μm and not more than 400 μm. Thereby, a silicon carbide single-crystal substrate capable of suppressing occurrence of a crack, and a method for manufacturing the same can be provided.

This application claims the benefit of U.S. Provisional Application No.61/622,082 filed Apr. 10, 2012, which is incorporated by referenceherein in the entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon carbide single-crystalsubstrate and a method for manufacturing the silicon carbidesingle-crystal substrate, and more particularly to a silicon carbidesingle-crystal substrate having a peripheral edge portion and a methodfor manufacturing the silicon carbide single-crystal substrate.

2. Description of the Background Art

A semiconductor substrate is manufactured, for example, by cutting acylindrical ingot with a wire saw, and grinding and polishing a surfaceand a peripheral edge portion of a cut substrate. Processing of theperipheral edge portion of the semiconductor substrate is calledbeveling processing or chamfering processing.

A method for processing a peripheral edge portion of a silicon wafer isdescribed for example in Japanese Patent Laying-Open No. 9-168947.According to the method, a peripheral edge portion of a silicon wafer isground by pressing a diamond grindstone against the peripheral edgeportion of the silicon wafer and simultaneously applying ultrasonicvibration to the diamond grindstone.

Further, a method for polishing a peripheral edge portion of a nitridesemiconductor substrate is described for example in Japanese PatentLaying-Open No. 2009-231833. According to the method, after a backsurface of a nitride semiconductor wafer is flattened, a process-damagedlayer produced at the back surface is removed by etching, and thereaftera peripheral edge portion of the wafer is ground with a diamondgrindstone.

SUMMARY OF THE INVENTION

In recent years, silicon carbide single-crystal substrates have comeinto use to manufacture semiconductor devices, considering advantagessuch as high breakdown voltage, low ON resistance, and the like. Siliconcarbide is harder than gallium nitride, and is difficult to be ground.There have been cases where, when a peripheral edge portion of a siliconcarbide single-crystal substrate is processed by the method described inJapanese Patent Laying-Open No. 2009-231833 and thereafter an epitaxiallayer is formed on the silicon carbide single-crystal substrate, a crackoccurs in the epitaxial layer.

The present invention has been made to solve the aforementioned problem,and one object of the present invention is to provide a silicon carbidesingle-crystal substrate capable of suppressing occurrence of a crack,and a method for manufacturing the same.

A mechanism causing occurrence of a crack will now be described. When asilicon carbide single-crystal substrate is subjected to bevelingprocessing, a process-damaged layer is formed in a chamfer portion ofthe substrate. The process-damaged layer refers to a layer having adisrupted crystal lattice. When a silicon carbide epitaxial layer isformed on the process-damaged layer, the epitaxial layer is grownabnormally because it is difficult to obtain lattice matching betweenthe process-damaged layer and the epitaxial layer, and a crack extendsin a direction of a main surface of the silicon carbide single-crystalsubstrate. If the crack grows, the silicon carbide single-crystalsubstrate may be broken. As a result of the earnest study of therelationship between the shape of the silicon carbide single-crystalsubstrate and occurrence of a crack, the inventors have found that theprobability of occurrence of a crack is influenced by a chamfer width ofthe silicon carbide single-crystal substrate. If the silicon carbidesingle-crystal substrate has a small chamfer width, an abnormally-grownregion of the epitaxial layer is close to the main surface of thesilicon carbide single-crystal substrate, and thus a crack is likely toextend in the direction of the main surface. On the other hand, if thesilicon carbide single-crystal substrate has a large chamfer width,there is a high probability that an edge portion may be chipped. If theedge portion is chipped, a crack occurs from the portion. Thus,occurrence of a crack can be suppressed by controlling the chamfer widthwithin a certain range.

A silicon carbide single-crystal substrate in accordance with thepresent invention includes a first surface, a second surface opposite tothe first surface, and a peripheral edge portion sandwiched between thefirst surface and the second surface. A plurality of grinding traces areformed in a surface of the peripheral edge portion. A chamfer width as adistance from an outermost peripheral end portion of the peripheral edgeportion to one of the plurality of grinding traces which is located onan innermost peripheral side of the peripheral edge portion in adirection parallel to the first surface is not less than 50 μm and notmore than 400 μm.

Here, being sandwiched between the first surface and the second surfaceincludes being sandwiched between a surface parallel to the firstsurface and a surface parallel to the second surface.

The silicon carbide single-crystal substrate in accordance with thepresent invention has a chamfer width of not less than 50 μm and notmore than 400 μm. Thereby, a silicon carbide single-crystal substratecapable of suppressing occurrence of a crack is obtained.

Preferably, in the silicon carbide single-crystal substrate, the surfaceof the peripheral edge portion has an arithmetic mean roughness of notless than 0.07 μm and not more than 3 μm. If the surface of theperipheral edge portion has a large arithmetic mean roughness, a crackis likely to occur because it is difficult to obtain lattice matchingbetween an epitaxial layer and the peripheral edge portion. By settingthe arithmetic mean roughness of the surface of the peripheral edgeportion to not less than 0.07 μm and not more than 3 μm, a siliconcarbide single-crystal substrate capable of suppressing occurrence of acrack is obtained.

Preferably, in the silicon carbide single-crystal substrate, theperipheral edge portion includes a process-damaged layer as a layerhaving a disrupted crystal lattice. The process-damaged layer has amaximum thickness of not less than 0.5 μm and not more than 10 μm. Ifthe process-damaged layer is thick, a crack is likely to occur becauseit is difficult to obtain lattice matching between the process-damagedlayer and the epitaxial layer. By setting the maximum thickness of theprocess-damaged layer to not less than 0.5 μm and not more than 10 μm, asilicon carbide single-crystal substrate capable of suppressingoccurrence of a crack is obtained.

A method for manufacturing a silicon carbide single-crystal substrate inaccordance with the present invention includes the steps of: preparing asilicon carbide single crystal having a pair of opposing main surfaces,and a peripheral edge portion sandwiched between the pair of mainsurfaces; preparing a grindstone having diamond abrasive grains embeddedin a binder, a hardness grade of the diamond abrasive grains and thebinder in accordance with the Japanese Industrial Standards being L toN, a degree of concentration of the diamond abrasive grains being notless than 80 and not more than 150; and polishing the peripheral edgeportion using the grindstone. Thereby, a silicon carbide single-crystalsubstrate capable of suppressing occurrence of a crack is obtained.

Preferably, in the method for manufacturing a silicon carbidesingle-crystal substrate, the diamond abrasive grains have a grain sizeof #400 to #2500 in accordance with the Japanese Industrial Standards.Thereby, a silicon carbide single-crystal substrate capable ofsuppressing occurrence of a crack can be obtained more accurately.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a configuration of asilicon carbide single-crystal substrate in accordance with oneembodiment of the present invention.

FIG. 2 is a cross sectional view schematically showing the configurationof the silicon carbide single-crystal substrate in accordance with oneembodiment of the present invention.

FIG. 3 is a plan view schematically showing the configuration of thesilicon carbide single-crystal substrate in accordance with oneembodiment of the present invention.

FIG. 4 is a view for illustrating a method for manufacturing the siliconcarbide single-crystal substrate in accordance with one embodiment ofthe present invention.

FIG. 5 is a view for illustrating the method for manufacturing thesilicon carbide single-crystal substrate in accordance with oneembodiment of the present invention.

FIG. 6 is an enlarged fragmentary view of a grindstone for use in themethod for manufacturing the silicon carbide single-crystal substrate inaccordance with one embodiment of the present invention.

FIG. 7 is a flowchart schematically showing the method for manufacturingthe silicon carbide single-crystal substrate in accordance with oneembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes an embodiment of the present invention withreference to the drawings. It is noted that in the below-mentioneddrawings, the same or corresponding portions are given the samereference characters and are not described repeatedly.

Further, regarding crystallographic indications in the presentspecification, an individual orientation is represented by [ ], a grouporientation is represented by < >, an individual plane is represented by( ) and a group plane is represented by { }. In addition, a negativeindex is supposed to be crystallographically indicated by putting “-”(bar) above a numeral, but is indicated by putting the negative signbefore the numeral in the present specification. For description of anangle, a system in which an omnidirectional angle is 360° is employed.

A silicon carbide single-crystal substrate in accordance with thepresent embodiment will be described with reference to FIGS. 1 and 2.

A silicon carbide single-crystal substrate 1 in accordance with thepresent embodiment includes a first surface 2A (front surface), a secondsurface 2B (back surface, and a surface opposite to the first surface),and a peripheral edge portion 9 and a body portion 7 sandwiched betweenfirst surface 2A and second surface 2B. Peripheral edge portion 9 has asurface 3 of peripheral edge portion 9, and a process-damaged layer 6.Process-damaged layer 6 refers to a layer having a disrupted crystallattice formed in a region of peripheral edge portion 9 close to surface3 by grinding or polishing a crystal. The existence and thickness ofprocess-damaged layer 9 can be confirmed by observing a cross sectionobtained by fracturing the crystal along a cleavage plane, through SEM(scanning electron microscope) observation, TEM (transmission electronmicroscope) observation, or cathodoluminescence observation.

Process-damaged layer 6 is formed from surface 3 of peripheral edgeportion 9 toward body portion 7 of silicon carbide single-crystalsubstrate 1. A thickness T of process-damaged layer 6 is at its maximumat an outermost peripheral end portion 4 on surface 3 of peripheral edgeportion 9, and becomes smaller toward first surface 2A and secondsurface 2B. Process-damaged layer 6 is formed in a circumferentialdirection of silicon carbide single-crystal substrate 1.

Referring to FIG. 3, in surface 3 of peripheral edge portion 9 ofsilicon carbide single-crystal substrate 1, a plurality of grindingtraces 5 a, 5 b, 5 c, and 5 d are formed substantially concentrically.Grinding traces 5 a, 5 b, 5 c, and 5 d are formed for example by diamondabrasive grains in a grinding step described later. The existence ofgrinding traces 5 a, 5 b, 5 c, and 5 d can be confirmed with an opticalmicroscope.

A region in which the grinding traces extending along thecircumferential direction are formed in surface 3 of peripheral edgeportion 9 of silicon carbide single-crystal substrate 1 is also referredto as a chamfer portion. In addition, a distance from outermostperipheral end portion 4 of peripheral edge portion 9 to grinding trace5 a located on an innermost peripheral side of peripheral edge portion 9in silicon carbide single-crystal substrate 1, in a direction parallelto first surface 2A, is referred to as a chamfer width L. In the presentembodiment, chamfer width L is not less than 50 μm and not more than 400μm.

First surface 2A of silicon carbide single-crystal substrate 1 ismirror-polished. Both first surface 2A and second surface 2B may bemirror-polished. Generally, however, the chamfer portion is notmirror-polished. Thus, it can be said that the chamfer portion (in otherwords, peripheral edge portion 9) is a portion serving as a pearskin ora semi-mirror surface, and first surface 2A is a portion serving as amirror surface.

Preferably, surface 3 of peripheral edge portion 9 of silicon carbidesingle-crystal substrate 1 has an arithmetic mean roughness (R_(a)) ofnot less than 0.07 μm and not more than 3 μm. Preferably,process-damaged layer 6 of peripheral edge portion 9 has a maximumthickness (T) of not less than 0.5 μm and not more than 10 μm. Inaddition, preferably, a value represented by R_(a)×T/L is not less than0.0000875 and not more than 0.6, where R_(a) represents the arithmeticmean roughness of surface 3 of peripheral edge portion 9 of siliconcarbide single-crystal substrate 1, T represents the maximum thicknessof process-damaged layer 6, and L represents the chamfer width.

Next, a method for manufacturing the silicon carbide single-crystalsubstrate in accordance with the present embodiment will be described.

First, a seed crystal made of single-crystal silicon carbide and sourcematerial powder made of silicon carbide are introduced into a cruciblemade of graphite. Next, by heating the source material powder, siliconcarbide is sublimated and recrystallized on the seed crystal. On thisoccasion, recrystallization proceeds while a desired impurity such asnitrogen is being introduced. Then, when a crystal of a desired size isgrown on the seed crystal, heating is stopped and a crystal ofsingle-crystal silicon carbide is taken out from a container.

Next, the fabricated crystal of single-crystal silicon carbide isprocessed into an ingot having, for example, a cylindrical shape. Theprocessed cylindrical ingot is set such that a portion of its sidesurface is supported by a support. Then, the ingot made of siliconcarbide single crystal is cut with a wire saw in a running direction tointersect with a <0001> direction. Thereby, silicon carbidesingle-crystal substrate 1 having a desired plane orientation is cutout. It is noted that silicon carbide single-crystal substrate 1 hasfirst surface 2A, second surface 2B opposite to the first surface, andperipheral edge portion 9 sandwiched between first surface 2A and secondsurface 2B.

Next, referring to FIG. 7, a grindstone preparation step is performed asstep (S10). In the grindstone preparation step, a grindstone for use inthe steps of grinding peripheral edge portion 9 of silicon carbidesingle-crystal substrate 1 described later is prepared.

Referring to FIG. 4, a grinding portion 10 has a grindstone 11 and arotary shaft 12 rotating grindstone 11 in an axial direction. Referringto FIG. 6, grindstone 11 has a base metal 20, a binder 21 provided onbase metal 20, and diamond abrasive grains 22 embedded in binder 21. Inthe present embodiment, a metal layer-type grindstone is prepared inwhich a hardness grade of diamond abrasive grains 22 and binder 21 inaccordance with the Japanese Industrial Standards is L to N, and adegree of concentration of diamond abrasive grains 22 is not less than80 and not more than 150. The metal layer-type grindstone refers to agrindstone having a binder made of a metal.

Preferably, the hardness grade of diamond abrasive grains 22 and binder21 in accordance with the Japanese Industrial Standards is M, and thedegree of concentration of diamond abrasive grains 22 is 125.

Here, the hardness grade is an index indicating the strength with whichbinder 21 holds the abrasive grains. The hardness grade is classifiedinto 26 types represented by A to Z of the alphabet, in which Arepresents the lowest hardness grade and Z represents the highesthardness grade. The hardness grade of L to N is classified as a mediumhardness grade. It is noted that details of the hardness grade isdescribed in the Japanese Industrial Standards R6240 and R6242.

Here, the degree of concentration is an index indicating how manyabrasive grains are contained in an abrasive grain layer. The case wherean abrasive grain percentage by volume is 25% (4.4 ct/cm³) is defined asthe degree of concentration of 100. For example, in the case where thedegree of concentration is 125, the abrasive grain percentage by volumeis 1.25 times higher (5.5 ct/cm³) than that in the case where the degreeof concentration is 100.

Diamond abrasive grains 22 for use in the present embodiment have agrain size of #400 to #2500. A grain size with a smaller numberindicates that the abrasive grains have a larger grain diameter. Theabrasive grains with a grain size of #400 have a mean grain diameter of37 μm, and the abrasive grains with a grain size of #2500 have a meangrain diameter of 6 μm. It is noted that details of the grain size isdescribed in the Japanese Industrial Standards R6001.

Next, a rough grinding step is performed as step (S20). In step (S20),peripheral edge portion 9 of silicon carbide single-crystal substrate 1is ground with a groove having a rough mesh size. Specifically, first,metal layer-type grindstone 11 prepared in step (S10), in which thehardness grade of diamond abrasive grains 22 and binder 21 is L to N,and the degree of concentration of diamond abrasive grains 22 is notless than 80 and not more than 150, is rotated about rotary shaft 12.For grindstone 11, diamond abrasive grains 22 having a grain size of#400 to #600 are used. Similarly, silicon carbide single-crystalsubstrate 1, which is an object to be ground, is rotated about a centralaxis of first surface 2A as a rotation axis. Silicon carbidesingle-crystal substrate 1 has a rotation speed (grinding speed) of notless than 1 mm/second and not more than 4 mm/second. Further, grindstone11 has a rotation speed in terms of the peripheral speed of not lessthan 1500 m/minute and not more than 3000 m/minute. As shown in FIG. 5,silicon carbide single-crystal substrate 1 moves in a direction Xparallel to first surface 2A and comes into contact with grindstone 11.Peripheral edge portion 9 of silicon carbide single-crystal substrate 1is ground by friction between grindstone 11 and silicon carbidesingle-crystal substrate 1. It is noted that a speed at which siliconcarbide single-crystal substrate 1 is pressed toward grindstone 11 andground in a radial direction of the substrate (cutting speed) is notless than 0.05 mm/second and not more than 0.3 mm/second.

In the rough grinding step, peripheral edge portion 9 of silicon carbidesingle-crystal substrate 1 is ground a plurality of times, and iseventually ground by 100 μm or more. However, the amount of grinding atone time is set to 100 μm or less.

Preferably, in the rough grinding step, diamond abrasive grains 22having a grain size of #600 are used. Preferably, the cutting speed is0.1 mm/second. Preferably, the grinding speed is 3 mm/second.Preferably, grindstone 11 has a rotation speed in terms of theperipheral speed of 2500 mm/minute.

Next, a finish grinding step is performed as step (S30). In step (S30),peripheral edge portion 9 of silicon carbide single-crystal substrate 1is ground with a groove having a fine mesh size. Specifically, first,metal layer-type grindstone 11 prepared in step (S10), in which thehardness grade of diamond abrasive grains 22 and binder 21 is L to N,and the degree of concentration of diamond abrasive grains 22 is notless than 80 and not more than 150, is rotated about rotary shaft 12.For grindstone 11, diamond abrasive grains 22 having a grain size of#800 to #2000 are used. Preferably, diamond abrasive grains 22 having agrain size of #1500 are used for grindstone 11. Similarly, siliconcarbide single-crystal substrate 1, which is the object to be ground, isrotated about the central axis of first surface 2A as the rotation axis.

The cutting speed, grinding speed, and grindstone speed in the finishgrinding step are the same as those in the rough grinding step.

In the finish grinding step, peripheral edge portion 9 of siliconcarbide single-crystal substrate 1 is ground a plurality of times, andis eventually ground by 100 μm or more. However, the amount of grindingat one time is set to 50 μm or less.

Through the above manufacturing method, silicon carbide single-crystalsubstrate 1 having the chamfer width described above of not less than 50μm and not more than 400 μm is obtained.

Next, function and effect of silicon carbide single-crystal substrate 1and the method for manufacturing the same in accordance with the presentembodiment will be described.

The silicon carbide single-crystal substrate in accordance with thepresent embodiment has a chamfer width of not less than 50 μm and notmore than 400 μm. Thereby, a silicon carbide single-crystal substratecapable of suppressing occurrence of a crack is obtained.

Further, the surface of the peripheral edge portion of silicon carbidesingle-crystal substrate 1 in accordance with the present embodiment hasan arithmetic mean roughness of not less than 0.07 μm and not more than3 μm. Thereby, a silicon carbide single-crystal substrate capable offurther suppressing occurrence of a crack is obtained.

Furthermore, the process-damaged layer of silicon carbide single-crystalsubstrate 1 in accordance with the present embodiment has a maximumthickness of not less than 0.5 μm and not more than 10 μm. Thereby, asilicon carbide single-crystal substrate capable of further suppressingoccurrence of a crack is obtained.

EXAMPLE

In the present example, the relationship among occurrence of a crackafter formation of an epitaxial layer on a silicon carbide semiconductorsubstrate, surface roughness R_(a) of the chamfer portion (peripheraledge portion 9) of silicon carbide single-crystal substrate 1, maximumthickness T of process-damaged layer 9, and chamfer width L wasinvestigated.

First, eight types of silicon carbide single-crystal substrates 1 havingsurface roughness R_(a) in a range of not less than 0.05 μm and not morethan 5 μm, maximum thickness T of the process-damaged layer in a rangeof not less than 0.4 μm and not more than 15 μm, and chamfer width L ina range of not less than 30 μm and not more than 500 μm were prepared.Silicon carbide single-crystal substrates 1 in accordance with thepresent invention's examples 1 to 6 were produced by the manufacturingmethod described in the embodiment. Next, a silicon carbide epitaxiallayer was formed on each silicon carbide single-crystal substrate 1. Theepitaxial layer was formed on first surface 2A and peripheral edgeportion 9 of silicon carbide single-crystal substrate 1. Formation ofthe epitaxial layer was performed at a temperature of not less than1500° C. and not more than 1650° C. and under reduced pressure. SiH₄/H₂was set to about 0.03%. The C/Si ratio was set to not less than 0.7 andnot more than 3.0. The film thickness of the epitaxial layer was set to15 μm.

Thereafter, it was evaluated whether or not the eight types of siliconcarbide single-crystal substrates 1 (the present invention's examples 1to 6, comparative examples 1 and 2) each having the epitaxial layerformed thereon had a crack. Observation of a crack was made with aNomarski microscope. The magnification of the Nomarski microscope wasset to 100, with the magnification of the eyepiece side being set to 10and the magnification of the objective side being set to 10. It is notedthat the microscope may be a stereoscopic microscope. In this case, acrack is observed by a reflection mode.

Observation of a crack was made by observing peripheral edge portion 9of silicon carbide single-crystal substrate 1 having grinding traces forchamfering. When there was a crack extending from peripheral edgeportion 9, the length of the crack in the direction of the main surface(direction of first surface 2A) was observed. In a case where a crackhaving a predetermined length or more was detected, it was determinedthat a crack was present, and in a case where a crack having thepredetermined length or more was not detected, it was determined that acrack was absent. Table 1 shows experimental results.

TABLE 1 Present Present Present Present Present Present Comparativeinvention's invention's invention's invention's invention's invention'sComparative example 1 example 1 example 2 example 3 example 4 example 5example 6 example 2 Chamfer portion surface 0.05 0.07 0.1 0.5 1 2 3 5roughness Ra [μm] Process-damaged layer thickness T 0.4 0.5 1 3 5 7 1015 [μm] Chamfer width L [μm] 500 400 300 200 150 100 50 30 Ra × T/L0.00004 0.0000875 0.000333 0.0075 0.033333 0.14 0.6 2.5 Presence/absenceof crack after Present Absent Absent Absent Absent Absent Absent Presentformation of epitaxial layer

As shown in Table 1, when chamfer width L was not less than 50 μm andnot more than 400 μm, a crack was not observed in silicon carbidesingle-crystal substrates 1 (the present invention's examples 1 to 6)each having the epitaxial layer formed thereon. On the other hand, acrack was observed in silicon carbide single-crystal substrate 1(comparative example 1) having chamfer width L of 500 μm and siliconcarbide single-crystal substrate 1 (comparative example 2) havingchamfer width L of 30 μm. In addition, when the chamfer portion hadsurface roughness R_(a) of not less than 0.07 μm and not more than 3 μm,a crack was not observed in silicon carbide single-crystal substrates 1each having the epitaxial layer formed thereon. Further, when theprocess-damaged layer had maximum thickness T of not less than 0.5 μmand not more than 10 μm, a crack was not observed in silicon carbidesingle-crystal substrates 1 each having the epitaxial layer formedthereon. Furthermore, when R_(a)×T/L had a value of not less than0.0000875 and not more than 0.6, and when chamfer width L was not lessthan 50 μm and not more than 400 μm, a crack was not observed in siliconcarbide single-crystal substrates 1 each having the epitaxial layerformed thereon.

As described above, it was confirmed that a silicon carbidesingle-crystal substrate capable of suppressing occurrence of a crack isobtained by adjusting chamfer width L within the above range.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the scopeof the present invention being interpreted by the terms of the appendedclaims.

What is claimed is:
 1. A silicon carbide single-crystal substrate,comprising: a first surface; a second surface opposite to said firstsurface; and a peripheral edge portion sandwiched between said firstsurface and said second surface, a plurality of grinding traces beingformed in a surface of said peripheral edge portion, a chamfer width asa distance from an outermost peripheral end portion of said peripheraledge portion to one of said plurality of grinding traces which islocated on an innermost peripheral side of said peripheral edge portionin a direction parallel to said first surface being not less than 50 μmand not more than 400 μm, wherein the peripheral edge portion includes aprocess-damaged layer as a layer having a disrupted crystal lattice, anda thickness of the process-damaged layer measured in a direction that isparallel to the first surface becomes smaller toward the first surface.2. The silicon carbide single-crystal substrate according to claim 1,wherein said surface of said peripheral edge portion has an arithmeticmean roughness of not less than 0.07 μm and not more than 3 μm.
 3. Thesilicon carbide single-crystal substrate according to claim 1, whereinsaid process-damaged layer has a maximum thickness of not less than 0.5μm and not more than 10 μm.
 4. The silicon carbide single-crystalsubstrate according to claim 1, wherein the process-damaged layer hasthe largest thickness measured in the direction that is parallel to thefirst surface at the center of the substrate, and the thicknesscontinuously decreases towards the first and second surfaces of thesubstrate.
 5. The silicon carbide single-crystal substrate according toclaim 4, wherein a maximal thickness of the process-damaged layermeasured in the direction that is parallel to the first surface is notless than 0.5 μm and not more than 10 μm.
 6. The silicon carbidesingle-crystal substrate according to claim 1, wherein theprocess-damaged layer has a sickled cross-sectional shape.